Macro fiber reinforced concrete (either steel or synthetic) enhances post‑cracking load capacity and toughness, providing an effective alternative to conventional reinforcement in many structural applications. This article outlines the design rationale behind reducing concrete thickness in elements such as slabs and linings by incorporating fibers; summarizes the influence of fibers on fresh and hardened concrete properties; and highlights key guidelines and references supporting performance‑based design.

Why is thickness reduction feasible?

• Fibers improve the residual flexural tensile strength of concrete, ensuring the element retains significant load‑bearing capacity even after cracking.
• They enhance crack control, toughness, and energy absorption, which allows for thinner cross‑sections in certain applications while maintaining performance.
• In some scenarios, macro fibers can partially or fully replace conventional temperature/shrinkage reinforcement, subject to project specifications and standards.

For systematic effects of fibers on fresh and hardened concrete, see FRCA’s Fibers in Practice note FIP‑7: Effect of Fibers on Concrete Properties. For design and specification perspectives, see FIP‑8: Design & Specification of Fiber‑Reinforced Concrete.

Key Fiber Effects Relevant to Thickness Reduction

• Toughness and post‑cracking behavior: Macro fibers significantly improve flexural toughness and residual strength, which is critical for safe performance of slender sections.
• Crack control and impact resistance: Numerous studies highlight improvements in impact and energy absorption capacity, particularly with hooked or deformed steel fibers.
• Workability: Higher fiber contents may reduce workability; optimized mix design and admixtures are necessary to ensure proper distribution and avoid clumping.

Design Logic Underpinning Thickness Reduction

Thickness reduction is only possible when load‑capacity and serviceability requirements are satisfied through validated post‑cracking behavior. Core steps include:

1. Material characterization through testing
   • Performance assessment involves residual strengths obtained via flexural tests (e.g., load–CMOD curves).
   • Guidelines: RILEM TC 162‑TDF σ–w method and σ–ε design method.
2. Section and load‑bearing verification
   • Both pre‑cracking (matrix strength) and post‑cracking (fiber contribution) behavior are considered.
   • Design checks cover ultimate limit states (bending, punching shear, impact) and serviceability (deflection, crack width, fatigue under repeated wheel loads).
3. Reinforcement substitution and optimization
   • FIP‑8 notes the growing role of fibers in replacing temperature/shrinkage reinforcement and, in some cases, even primary reinforcement.
   • When required residual strength classes (fR values) are achieved, slab thickness can be safely reduced.
4. Comparative assessment vs. conventional rebar
   • FIP‑9 provides key decision criteria for choosing between fibers and conventional mesh reinforcement in specific applications.

Applications with Higher Potential for Section Thinning

• Industrial floors / slabs‑on‑ground: Optimized slab thickness can be achieved when fiber performance is validated, accounting for wheel loads, rack loads, subgrade modulus, and joint spacing.
• Shotcrete and tunnel linings: Improved energy absorption enables thinner support linings, provided residual strength parameters are verified.
• Precast panels and architectural elements: Enhanced toughness and crack control allow for more slender components without compromising safety.

References: FRCA FIP‑7, FIP‑8; RILEM 1337 (σ–w method); RILEM 1507; Discover Materials 2024 Review.

Practical Design Roadmap

1. Define performance objectives (ULS vs. SLS).
2. Obtain tested residual strengths appropriate to the chosen fiber type and dosage.
3. Apply validated design methods (RILEM TC 162‑TDF).
4. Optimize thickness and/or reinforcement substitution using performance graphs (fiber dosage vs. residual strength).
5. Ensure proper execution on site: optimized mix, uniform fiber dispersion, proper finishing and curing, adjusted joint layout.

Sources for Further Reading

• FRCA Fibers in Practice series:
  • FIP‑7: Effect of Fibers on Concrete Properties
  • FIP‑8: Design & Specification of FRC
  • FIP‑9: Fibers vs. Conventional Reinforcement
• RILEM 1337 PDF – σ–w Method
• RILEM 1507 PDF – Test & Design Methods

Discover Materials (2024) – State of the Art Review

Conclusion

Macro fiber reinforcement provides a credible pathway to reducing concrete thickness in slabs and linings by offering enhanced post‑cracking residual capacity and toughness. However, this potential requires test‑based calibration and design procedures grounded in international guidelines such as RILEM TC 162‑TDF and FRCA’s Fibers in Practice notes. In this framework, fibers move beyond crack control, enabling optimized, leaner, and more durable structural solutions.